The use of electrically self-heated resistors, hot wires and hot films, as anemometer transducers is well known in the prior art. In such devices, a heated resistance element serves as a sensing element, and its physical geometry is used to define its spatial response to impinging fluid flow. Most widespread use has been in the measurement of airflow. The sensing element has a non-zero temperature coefficient of resistance and is maintained at constant resistance and, thus, constant temperature while it is operated as part of a feedback controlled electrical bridge circuit. An example of such a prior art constant temperature anemometer circuit is illustrated in FIG. 1 wherein a single sensing element 10 forms one arm of a four arm Wheatstone bridge which is completed by resistances 11, 12 and 13. Differential amplifier 14 is connected to the bridge at points 15 and 16 in order to determine bridge balance or bridge error signal. Amplifier 14 output 17 is fed back to the top of the bridge at the junction of resistors 11 and 13 in order to provide bridge excitation. For clarity's sake, power supply connections are not shown in this figure.
FIG. 2 illustrates the signal form seen at point 17 of FIG. 1. The resulting signal output 17 is unipolar and is markedly non-linear, containing three components: an approximate fourth root term as a function of mean flow, a turbulence component which results from fluctuations in the flow, and a d-c or constant term 19 which is the zero flow quiescent heating signal developed across the complete bridge. Again, referring back to FIG. 1, in the art it is customary for the anemometer output signal to be taken at point 17, the low impedance output of amplifier 14, at the top of the bridge with resistor 11 connected in series with sensing element 10. A more accurate but lower level output can also be taken across sensing element 10, from point 15 to ground 18. Use of output 17 or the potential across resistor 10 are classic ways to read out a single-ended or unbalanced output signal from a constant temperature anemometer. The desired parts of the output signal are mean flow and turbulence components. The constant term 19 or zero offset portion is of little interest. The turbulent component can easily be separated by use of an a-c coupled amplifier, but it is more difficult to separate mean flow from the d-c or constant term 19. An opposing bias voltage is customarily used to offset or balance out the zero flow output value of mean flow. Examples of single-ended unipolar constant temperature anemometer transducers, together with bridge operating circuits therefor, are shown in U.S. Pat. Nos. 3,220,255; 3,352,154; 3,363,462; 3,900,819; 3,991,624; 4,373,387; 4,503,706 and 4,523,462.
The research anemometer user customarily recalibrates his instruments with each use, resets offset or bias adjustments as needed, and his investigations are usually not long in duration. Long term unattended anemometer use is another matter entirely. Temperature drift and stability of bias and offset reference potentials introduce errors and limit the usefulness of the constant temperature anemometer as a field research tool. Temperature compensation of the anemometer is accomplished by using resistor 12 in FIG. 1 to sense ambient temperature and adjust the operating point of sensing element 10. A resistance temperature detector (RTD) or a resistance thermometer or similar temperature sensitive resistor is used in place of fixed low temperature coefficient resistor 12 in order to sense and track ambient temperature variations and automatically adjust anemometer sensitivity to variations in temperature. A detailed discussion of this technique is presented in U.S. Pat. No. 3,363,462, and Sabin's design equations are particularly useful for thermal anemometry generally. A simplified discussion of the technique of temperature compensation is later given in U.S. Pat. No. 4,794,795.
A detailed discussion of the constant temperature anemometer can be found in pages 59 through 92 and 172 through 176 of a book entitled "Hot-wire Anemometry" by A. E. Perry, published in 1982 by Oxford University Press, New York, ISBN 0-19-856327-2. Thermal anemometers are also well described in chapter four, pages 99 through 154, of a book entitled "Fluid Mechanics Measurements", edited by Richard J. Goldstein and published in 1983 by Hemisphere Publishing Corporation, New York, ISBN 0-89116-244-5.